//对象转查询字符串
function objToSearchString(obj){
    var str  = ''
    for(var key in obj){
        str += `${key}=${obj[key]}&`;
    }
    return str.slice(0,str.length-1);
}

//封装ajax函数
function ajax( method, url, params ){
   return new Promise((resolve, reject) => {
     //1.创建请求对象
     var xhr = new XMLHttpRequest()

     //2.配置请求对象
     xhr.open(method, method == 'get' ? url + "?" + objToSearchString(params) : url );
 
     //3.设置回调函数
     xhr.onreadystatechange = function(){
         if( xhr.readyState == 4 ){
             resolve( JSON.parse(xhr.responseText) )
         }
     }
 
     //4.发送请求
     //先对post请求设置 content-type 请求头
     xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');
     xhr.send(  method == 'get' ? null : objToSearchString(params) );
   })
}


